1. Technical Field
The present invention relates generally to an ignition apparatus, and, more particularly, an ignition apparatus having an electrically floating shield.
2. Discussion of the Background Art
Ignition apparatuses utilize primary and secondary windings and a magnetic circuit. The magnetic circuit may include a central core formed of steel laminations or compression molded insulated iron particles, and a side core or shield, tubular in shape, formed of silicon steel, as seen by reference to U.S. Pat. No. 5,706,792 issued to Boyer et al. Boyer et al. further disclose an ignition apparatus having a relatively slender configuration adapted for mounting directly above a spark plug in a spark plug wellxe2x80x94commonly referred to as a xe2x80x9cpencilxe2x80x9d coil. Boyer et al. further disclose that the shield is electrically grounded, ostensibly to inhibit a voltage rise from occurring at the shield. Boyer et al. further disclose that the shield is the radially outermost portion of the ignition coil (i.e., it has no electrical insulation outwardly thereof).
The ignition apparatus of Boyer et al. is of the type having a secondary winding that is outwardly of the primary winding. This type yields a relatively high electric field between the secondary winding and the shield. This electric field, among other things, results in a relatively high capacitance with respect to the secondary winding. The secondary winding voltage that can be obtained during operation is determined in terms of energy and capacitance, as follows:
V={square root over (2*E/C)}
In order to obtain a short charge time and a low energy per pulse, for example as may be desirable in a multicharge/multistrike system, the low energy may not be able to charge the secondary winding capacitance to an acceptable value. This situation is generally undesirable.
There is therefore a need for an ignition apparatus that minimizes or eliminates one or more of the problems as set forth above.
An object of the present invention is to solve one or more of the problems set forth in the Background. One advantage of the present invention is that it provides an ignition apparatus having a secondary capacitance that is reduced relative to conventional ignition apparatuses, which allows a relatively short charge time. In addition, the reduced capacitance results in a reduced electric field between the secondary winding and the shield assembly (i.e., a reduced electric field through the case), thereby increasing durability of the ignition apparatus (i.e., a high electric field tends to break down common case materials due to corona discharge erosion).
An ignition apparatus according to the invention comprises a core having a main axis, a primary and secondary winding radially outwardly of the core, a first end of the primary winding being configured to be coupled to a power source and a second end coupled to ground, a conductive shield radially outwardly of the windings that is floating relative to ground, and a housing of electrically insulative material radially outwardly of the shield. Since the shield is allowed to float, the capacitance between the secondary winding and the shield drops by about four times. This reduces the overall capacitance seen by the secondary winding by between about 20% to 30% relative to conventional configurations where the shield is grounded. The shield, being ungrounded, however, rises to a voltage of about xc2xd the secondary voltage. During discharge (i.e., spark event) this level may be relatively high. The insulative housing inhibits arcing between the shield and a local ground (e.g., part of the spark plug well).
As an optional feature, the ignition apparatus further includes a boot, made of electrical insulative material, overlapping an interface where an axial end of the case engages the housing.